Convergent boundaries: subduction and collision
The Andes, the island arcs of Japan and the Zagros of Iraq and Iran.
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What you see
Choose the plates. Ocean–continent: the slab sinks, earthquakes trace it into the mantle and a volcanic mountain range rises. Ocean–ocean: volcanoes grow from the sea floor into an island arc. Continent–continent: the Neo-Tethys closes, Arabia meets Iran, the layers fold into the Zagros, ophiolites are pushed onto the margin and the crust grows a deep root.
Key ideas
- Denser oceanic lithosphere sinks beneath lighter plates at trenches; continental lithosphere resists sinking.
- Water released from the slab at about 100 km depth melts the mantle wedge and feeds arc volcanoes.
- Earthquakes along the slab (Wadati–Benioff zone) reach about 700 km.
- The Zagros fold-thrust belt formed by the collision of Arabia with Iran after the Neo-Tethys closed; its anticlines hold the oil fields of Kurdistan and Iran.
Test yourself
1. Why does the oceanic plate sink beneath the continent?
Old oceanic lithosphere is cold and dense; continental crust is buoyant.
2. What melts the mantle under a volcanic arc?
Water lowers the melting temperature of mantle rock (flux melting).
3. Which collision built the Zagros mountains?
Arabia moved north, consumed the Neo-Tethys and collided with the Iranian plate.
References
- Stern, R.J. (2002). Subduction zones. Reviews of Geophysics, 40(4), 1012.
- Kearey, P., Klepeis, K.A. & Vine, F.J. (2009). Global Tectonics, 3rd ed. Wiley-Blackwell.
- Alavi, M. (2004). Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. American Journal of Science, 304, 1–20.
- Jassim, S.Z. & Goff, J.C. (eds.) (2006). Geology of Iraq. Dolin, Prague and Moravian Museum, Brno.